The present invention relates to an exhaust gas purification system for purifying the exhaust gas emitted from the internal-combustion engine of automobiles and the like by removing hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) of the exhaust gas.
Japanese Patent First Publication JP-A-5-200287 discloses a three-way catalyst. JP-A-5-49929 discloses a heat resistant three-way catalyst.
In case of an exhaust gas purification system having at least one upstream catalyst and at least one downstream catalyst, which are arranged in line in an exhaust gas passage of the system, hydrocarbons of the exhaust gas are allowed at first to flow through the upstream catalyst and then through the downstream catalyst. In view of this fact, the inventor unexpectedly found that most of easily oxidizable hydrocarbons are oxidized or removed by the upstream catalyst, and then an exhaust gas having a high content of slightly oxidizable hydrocarbons (e.g., saturated hydrocarbons) is allowed to flow through the downstream catalyst. Thus, he unexpectedly found that conversion of hydrocarbons by the downstream catalyst tends to be lower than that of hydrocarbons by the upstream catalyst. Then, he recognized a need for an exhaust gas purification that can sufficiently oxidize or remove even slightly oxidizable hydrocarbons.
It is therefore an object of the present invention to provide an exhaust gas purification system that is capable of removing hydrocarbons with high efficiency.
It is a more specific object of the present invention to provide an exhaust gas purification system that can sufficiently oxidize or remove even slightly oxidizable hydrocarbons.
According to the present invention, there is provided an exhaust gas purification system for removing carbon monoxide, hydrocarbon and nitrogen oxide from an exhaust gas of an internal combustion engine. This system comprises: an upstream catalyst comprising at least one selected from the group consisting of platinum, palladium and rhodium; and a downstream catalyst that is arranged downstream of the upstream catalyst. This downstream catalyst comprises platinum in an amount greater than that of the platinum of the upstream catalyst. In the system, a first oxygen excess ratio of an exhaust gas at an inlet of the upstream catalyst is adjusted to at least 0.9, and a second oxygen excess ratio of an exhaust gas at an outlet of the upstream catalyst and at an inlet of the downstream catalyst is adjusted to at least 0.1.